This invention involves the printing of images by more than one complete pass across a line of printing when this is necessary to counter the effects of residual heat in the printhead. Such multiple pass printing to avoid overheating the printhead is known prior to this invention in which a column of print elements is driven in either two or three passes as described in IBM Technical Disclosure Bulletin article entitled "All-Points-Addressable Printing With A Resistive Ribbon," Vol. 29, No. 2, July 1986 at pp. 609-610. The article discloses resistive ribbon printing of the kind employed in the preferred embodiment of this invention with a vertical column of 40 electrodes used for printing each line.
To print all-points-addressable graphics in that prior system the 40 electrodes are not driven simultaneously regardless of the content of the data to be printed. Instead, at least two passes of the electrodes are made in the printing of each line, with provision for three passes when the data is particularly heavy in current-intensive, black elements, typically dark or black areas. In the two pass operation, alternating groups of two adjacent electrodes are driven to the extent that the content of the data calls for the driving of those electrodes. The printhead is returned to the start of the line and the line is traversed on a second pass with the remaining groups of two adjacent electrodes driven to the extent that the content of the data calls for the driving of those electrodes. Accordingly, at the end of two passes, each electrode has been driven with the data and at the position on the paper being printed as would have been the case had all 40 electrodes been operated on in a single pass in accordance with the original graphic data to be printed.
The data of the line is examined in continuous segments or blocks. Where such an interval of data is above a given percentage of black content, a three pass mode is entered. In the three pass mode two adjacent electrodes are driven in the first pass to the extent that the content of the data calls for the driving of those electrodes, while the next four electrodes are not driven. The remaining electrodes in the column are driven according to the same pattern of two adjacent electrodes driven and the next four not driven. In the second pass, two adjacent electrodes in each group of the four previously not driven are driven to the extent that the content of the data calls for the driving of those electrodes. In the third pass, the remaining two electrodes in each of the groups of six electrodes are driven to the extent that the content of the data calls for the driving of those electrodes. Accordingly, at the end of the three passes, each electrode has been driven with the data and at the position of paper being printed on as would have been the case had all 40 electrodes been operated in one pass in accordance with the original graphic data.
U.S. Pat. No. 4,454,516 to Moriguchi et al discloses a thermal printing system in which thermal printing in one line is carried out in one or more operations dependent upon the amount of printing called for by the data. U.S. Pat. No. 4,567,488 to Moriguchi et al is a prior teaching of general interest in that it is illustrative of a thermal printing system in which the content of data defining printing near an element to be driven, including both before and after an element to be driven, is used to define the extent to which the element is driven.
This invention employs the examination of continuous intervals or segments of data in a line with the printing of all data in the interval except where the examination indicates a predetermined concentration of heat-intensive, high density (typically black) elements. At such a concentration, the next interval is not printed until a second pass. The following interval is normally then printed in full. Thus, each interval is printed using all the electrodes simultaneously to the extent the content of the data calls for driving all of those electrodes; although where the data is sufficiently heat-intensive, some intervals are omitted in the first pass.